In order to reduce unwanted volatile organic compounds such as solvents in the printing machine room, printing inks on a water base corresponding to printing inks on an organic base, i.e., an oil base, are increasingly preferred. Water free inks without organic bases are even more preferred. A water free printing ink which can be washed off without the help of organic solvents which emit volitile compounds is needed in the industry.
There are known waterbased printing inks for printing processes, especially for flexographic printing processes and intaglio printing processes, in which the areas of the printing plate which are supposed to take up the printing ink are characterized by depressions of the printing plate (relief process). The lithographic printing process has always presented unique challenges to ink formulators, since such process utilizes a planographic printing plate, wherein the image and non-image areas are in the same plane on the image carrier, and two fluids are concurrently utilized to insure that ink adheres only to the image area, and not to the non-image area. In conventional lithographic printing processes, the plate is damped before it is inked with an oil based ink. Typically, the damping process utilizes a water or aqueous fountain solution, such as those described in U.S. Pat. Nos. 3,877,372; 4,278,467; and 4,854,969. Upon damping, water forms a film on the hydrophilic areas (the non-image areas) of the printing plate, but contracts into tiny droplets on the oleophilic areas (the image areas) of the plate. When an inked roller containing the oil based ink is passed over the damped plate, it is unable to ink the areas covered by the water film (the non-image areas), but will emulsify the droplets on the water repellant areas (the image areas), causing such area to ink up. In the process of “offset lithography,” the inked image on the plate does not directly print onto the paper substrate, but is first “offset” onto a rubber blanket, then transferred therefrom onto the paper substrate. Establishing and maintaining a correct ink/water balance during the printing process is critical, and requires a high level of skill. This is one of several disadvantages associated with such printing processes, as compared to flexographic and gravure printing processes. Moreover, the oil based inks and aqueous fountain solutions typically employed in conventional offset lithographic printing processes contain fairly high levels of undesirable VOCs, generally from 30 to 45%(is it that high). Further, the cleaning solutions used for press wash up may also contain VOCs.
Efforts have been undertaken to develop waterless lithographic printing processes. In these, the nonimage areas of the printing plate are coated with a polymer that repels the printing ink. The image areas are coated, for example, with a photosensitive polymer. Such printing processes are known, for example, from U.S. Pat. No. 5,370,906 and U.S. Pat. No. 5,417,749. The drawback to the known waterless printing processes is that the difference in surface energy between image area and nonimage area is drastically reduced, namely, from around 40 mN/m to 20 mN/m, thus resulting in decreased resolution of the ink. In order to preserve a large difference in surface energy between image area and nonimage area, which is important for good resolution, only the use of oil-based printing inks thus remains and thus the necessity of organic cleaning solvents remains.
From WO 96/34923 there is known a printing ink whose binding agent is formulated on an oil base and which can be washed off with an aqueous-alkaline medium. The drawback to this formulation is that it is not entirely water-washable and it contains free carboxyl groups.
Other approaches have suggested the use of a humidity chamber (U.S. Pat. No. 5,725,646) or special rewetting agents to prevent the evaporation of water from the printing rollers (U.S. Pat. No. 6,444,021). The rewetting agents affect the printability of the ink, as they do not dry effectively without heat and are therefore more costly to print with. In addition these compositions require high concentrations of humectant or rewetting agent to achieve the desired tack stability for the ink. Further, in U.S. Pat. No. 6,444,022 the water based lithographic ink uses certain linseed oil modified resins and humectants which extinguished the need for the humidity chamber associated with waterbased inks described in U.S. Pat. No. 5,725,646, but the inks retained gloss and drying problems. Further, these inks contain water which also affects printing qualities.
Today, organic detergents and/or solvents are primarily used for cleaning machine parts and mechanisms that have taken on ink during the printing. The drawback to the organic detergents is that they often liberate toxicologically unsafe volatile organic compounds.
Accordingly there is a need for a waterless water washable composition that uses no humectants and does not give off organic volatile compounds during its use while still maintaining suitable tack and viscosity for use in lithographic printing.